The present invention relates, in general, to semiconductor pressure transducers, and more particularly, to a piezoresistive transducer with an output voltage that exhibits low drift.
Piezoresistive transducers had long been produced by the semiconductor industry. These transducers generally were resistors that were formed on a silicon die that had a thin flexible silicon diaphragm. The silicon diaphragm would flex in response to a differential pressure between the die's two principle surfaces. As the silicon diaphragm flexed, the resistor also flexed thereby changing the resistor's value. Piezoresistive transducers typically had four terminals and functioned as a Wheatstone bridge. A reference voltage was applied to two terminals of the piezoresistive transducer, and a sense voltage was measured between two other terminals. As a differential pressure flexed the resistor and changed its resistance, the sense voltage changed in proportion to the pressure differential. An important parameter of piezoresistive transducers was offset voltage. Offset voltage was the value of the sense voltage at a differential pressure of zero. Previous piezoresistive transducers' offset voltage changed over a period of time, or drifted. That is, the measured value of the offset voltage drifted or changed without a change in differential pressure. In addition, the drift increased with increased humidity and temperature. Although drift was generally measured at a differential pressure of zero, the drift not only affected the offset voltage, but also the output voltage at other differential pressures. The output voltage variations were not consistent and the amount of variation could be different for different measurements at different temperatures and humidity. This drift characteristic limited the accuracy of the transducers and limited the useful applications of piezoresistive transducers.
Accordingly, it would be desirable to have a piezoresistive transducer that has low drift.